The
posterior wall of middle ear cavity (Tympanum) is also known as
retrotympanum. Important anatomic structures are lodged in this area.
This area has assumed significance because of the difficulties
encountered in clearing cholesteatoma from this area. This area is so
narrow and has lot of crevises, it is very difficult to clear disease
from this area.

This
area is supposed to contain 4 important recesses. Each of these four
recesses could hide cholesteatoma causing the surgeon to leave residual
disease which could later recur. Precise knowledge of anatomy of this
region is vital for the surgeon who wants to clear disease from this
area. The recesses present in the retrotympanic area are:

Sinus tympani

Lateral tympanic sinus

Posterior tympanic sinus

Facial recess

Pyramidal
eminence is the most prominent anatomical landmark of this area. This
eminence hold the pyramidalis muscle. There are other prominences
arising from this area projecting in various directions. They include:

External: Chordal ridge

Inferior: Pyramidal ridge

Superior: Suprapyramidal ridge

Internal: Ponticulus

The 4 types of retrotympanic recesses are found under these eminences.

Sinus tympani:

This
is the most common and constant depression present in the retrotympanic
area. Anatomically this sinus is located at the junction of the lateral
and posterior walls of the tympanic cavity. Phylogentically this recess
is considered to ba analogue of bulla tympanica seen in mammals. It
lies between ponticulus superiorly and subiculum inferiorly. This
recess is bounded by pyramidal ridge externally and promontory
internally. Visualization of this area during middle ear surgery proves
to be a challenge. During yester years small angled mirrors known as
zinne mirrors were used. Now angled telescopes serves this function
rather brilliantly. The sinus tympani is known to extend posteriorly up
to the round window niche.

Types of sinus tympani:

Sinus
tympani has been classified into three types depending on its depth.
Note in type III it extends up to the level of lateral semicircular
canal.

Figure showing the types of sinus tympani

Figure showing anatomy of sinus tympani

Lateral tympanic sinus:

Proctor described this sinus in 1969. This sinus lies between three eminences of styloid prominence. These eminences include:

Pyramidal eminence

Styloid eminence

Chordal eminence

Posterior tympanic sinus:

Posterior sinus of middle ear cavity is one of the recently identified anatomical sinus inside the middle ear cavity.Serial temporal bone dissections have shown that it is present in nearly 90% of dissected bones.

Position: It lies just posterior to the oval window.

Depth: 1mm or lessWidth: 1.5 mm or less

In nearly 60% of dissected specimen a ridge of bone arising from the floor of middle ear cavity separates it from sinus tympani.

In 8% of dissected specimen, the sinus tympani and posterior sinus merged together to form one confluent sinus.

It
has been demonstrated that cholesteatoma / granulation tissue may lie
within this sinus making removal difficult leading on to residual
disease.Retraction pockets may also occur close to this area.

Figure showing posterior sinus

Facial recess:

This
recess lies between the promontory and tympanic annulus. It is bounded
medially by the facial nerve and laterally by tympanic annulus. Running
between these two structures at varying angulations is the chorda
tympani nerve. Chorda tympani nerve always runs medial to the ear drum.
Drilling in this area between the facial nerve, annulus and the angle
formed by the chorda tympani nerve will lead into the middle ear cavity
without causing a breach in the ear drum. This approach is used in
cochlear implant surgery to place the electrode in the round window
area. Hypotympanum can also be approached through this approach.

Figure showing middle ear anatomy

Subiculum:

This is the posterior extension of promontory separating oval and round windows.

Ponticulus:

Rarely
a spicule of bone arises from the promontory above the subiculum and
runs to the pyramid on the posterior wall of the middle ear cavity.
This spicule of bone is known as the ponticulus.